In recent years, biodegradable plastics have been actively developed as materials that can solve problems caused by plastic waste that places a heavy burden on the global environment, such as impact on the ecosystem, generation of harmful gases during combustion, and global warming due to a large amount of heat generated by combustion.
In particular, carbon dioxide generated by combustion of plant-derived biodegradable plastics was originally present in the air, and therefore the amount of carbon dioxide in the air does not increase. This is referred to as carbon neutral, and is regarded as important under the Kyoto Protocol that sets carbon dioxide reduction targets. For this reason, biodegradable plastics have been expected to be actively used.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester resins, especially polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA) resins have received attention as plant-derived plastics. Among PHA resins, for example, poly(3-hydroxybutyrate) homopolymer resins (hereinafter, sometimes referred to as P3HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resins (hereinafter, sometimes referred to as P3HB3HV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resins (hereinafter, sometimes referred to as P3HB3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resins (hereinafter, sometimes referred to as P3HB4HB), and polylactic acid have received attention.
However, such PHA resin has a slow crystallization speed, and therefore its molding processing requires a long cooling time for solidification after heat-melting, which causes problems such as poor productivity and temporal change in mechanical properties with passage of time (especially, toughness such as tensile elongation at break) due to secondary crystallization that occurs after molding.
In order to solve such problems, addition of an inorganic substance such as boron nitride, titanium oxide, talc, lamellar silicate, calcium carbonate, sodium chloride, or metal phosphate to the PHA resin has heretofore been proposed to promote crystallization. However, addition of such an inorganic substance has many negative effects such as reduction in the tensile elongation of a resulting molded article, and poor appearance of the surface of the molded article, and therefore its effect is inadequate.
Other methods have been proposed to promote the crystallization of the PHA resin. Examples of such methods include: one in which a natural product-derived sugar alcohol compound such as erythritol, galactitol, mannitol, or arabitol is added (PTL 1); and one in which polyvinyl alcohol, chitin, or chitosan is added (PTL 2). However, the fact is that a practically effective crystal nucleating agent has not yet been found.